Apps on modern mobile operating systems can access various system resources with, or without, an explicit user permission.  Although the OS generally maintains strict separation between apps, an app can still get access to another app's private information, such as the user input, through numerous side-channels.  For example, keystrokes and swipe gestures from a victim app can be inferred indirectly from the accelerometer or gyroscope output, allowing a zero-permission app to learn sensitive inputs such as passwords from the victim's app.  Current mobile OSes allow an app to defend itself in such situations only in some exceptional cases - e.g., by blocking screenshot captures in Android.

In this article, we propose a general mechanism for apps to defend themselves from any unwanted implicit or explicit interference from other concurrently running apps.  Our AppVeto solution enables an app developer to easily configure an app's requirements for a safe environment; a foreground app can request the OS to disallow access - i.e., to enable veto powers - to selected side-channel-prone resources to all other running apps for a certain (short) duration, e.g., no access to the accelerometer during password input.  In a sense, we enable a finer-grained access control policy than the current runtime permission model.  We implement AppVeto on Android using the Xposed framework and Procedure Linkage Table hooking techniques, without changing Android APIs.  Furthermore, we show that AppVeto imposes negligible overhead, while being effective against several well-known side-channel attacks - implemented via both Android Java and/or Native APIs.